[MouseEventListener] Allow accepting mouse event

This will keep the event from propagating. Accepting a press event will also not result in
clicked or pressAndHold being handled.

In Qt 5.10 event propagation changed resulting in FolderView opening both the item context menu
and containment context menu. Imho this actually makes sense since we never accepted the
mouse event there.

Differential Revision: https://phabricator.kde.org/D8864